Add a pause/stop API

[jnm2]

Tenet: privacy

Alternatives: report windows (top-level and child) to blur instead of stopping everything on-screen?

It doesn't seem ideal to erase all buffered frames when you pause, so that means that a GIF will need a frame to be inserted for the duration of the pause. It should be self-explanatory so that it doesn't appear that this is what the app actually looked like at the time or that the recording itself failed.